In recent years various types of silicon pressure transducers which employ a monocrystaline silicon wafer as a diaphragm have been proposed. These transducers contain diffused elongated resistances, which are obtained through present day integrated circuit technology and employ a characteristic of silicon in that its specific resistance changes due to strain, i.e. the piezo-resistive effect. The magnitude of the piezo-resistive effect differs greatly in dependence upon the crystal axis direction. The degree of strain of a silicon diaphragm in response to the application of pressure will differ for respective positions on the diaphragm.
One of the requirements of a silicon pressure transducer is that when the bridge is composed of elongated resistances, the pressure-electric output transduction sensitivity should be high. Moreover, when the pressure is zero, there should be no electric output and, furthermore, the pressure transducer should be minimally susceptible to the influence of changes in ambient temperature.
U.S. Pat. No. 3,537,319 to Yerman, issued Nov. 3, 1970 discloses one type of prior art silicon pressure transducer. In accordance with the pressure transducer described in the patent, when pressure is applied on one side of the silicon diaphragm, a compressive strain is created at the central portion of the diaphragm, whereas a tensile strain is created in the portion of the diaphragm surrounding the central portion and close to the edge of the strain inducing region. As a result, along the radius of the central portion and the surrounding portion close to the edge of the strain inducing region, p-type elongated resistances are diffused. These resistances are formed in the &lt;111&gt; direction of the (110) face of an n-type silicon diaphragm. The &lt;111&gt; direction is employed since the strain-resistance transduction sensitivity (the gaugefactor) in this direction is the greatest.
In accordance with this transducer, however, the four elongated resistances which are combined into a bridge are separately arranged in two portions of the diaphragm. Two of the resistances are disposed in the central portion of the diaphragm while the remaining two resistances are disposed in the surrounding portion of the diaphragm. As a result, variations in the dimensions and impurity concentrations of the individual resistances are prone to occur during the process of forming the respective resistances, particularly during the photo-etching and impurity diffusion steps. As a result, there will necessarily occur variations of the resistance values and temperature coefficients of the respective elongated resistances. This means that when the bridge is formed of these elongated resistances, it necessarily becomes an unbalanced bridge and will provide an electric output even for zero pressure input; also, the electric output is affected by the ambient temperature.
In an attempt to minimize and prevent the occurrence of such possible variations in the resistances contained within a semiconductor pressure transducer, the present inventors approached the problem in forming four elongated resistances in proximity to one another. An attempt was made to provide p-type elongated resistances in proximity to one another, the individual resistances being disposed only within the tensile strain region of the surrounding portion of the n-type silicon diaphragm, of the type described in the above-referred to patent. Two elongated resistances which were to form one set of opposing arms of the bridge were arranged in the &lt;110&gt; axial direction of a radial direction of the crystal of the (100) face of the n-type silicon diaphragm. The remaining two resistances which were to form the other set of resistances of the opposing arms of the bridge were disposed in the &lt;110&gt; axis direction intersecting in a direction orthogonal to the direction of the first mentioned pair of resistances. Namely, in the (110) face of the n-type silicon diaphragm, one pair of opposing arms of the bridge, made up of the respective elongated resistances, were disposed in the &lt;111&gt; axial direction while the other two resistances were formed in the &lt;112&gt; axial direction, intersecting orthogonally to the former direction.
With this arrangement, the inventors determined that the strain-resistance transduction sensitivity was reduced in dependence upon the orientation of the elongated resistances. The impurity diffusion rate differs in accordance with the direction of the silicon crystal axis, so that the dimensions of the elongated resistances in accordance with the different crystal directions were necessarily not the same and exhibited different resistance values. Consequently, when the bridge was assembled with these elongated resistances, the bridge was unbalanced and an electric output was provided even for zero pressure input to the silicon diaphragm.